7.2.4. Extreme Weather Events

Many climatic impacts are related to extreme weather events, and the same will
hold for the impacts of climate change. The large damage potential of extreme
events arises from their severity, suddenness, and unpredictability, which makes
them difficult to adapt to. Development patterns can increase vulnerability
to extreme events. For example, large development along coastal regions increases
exposure to storm surges and tropical cyclones, increasing vulnerability.

The frequency and magnitude of many extreme climate events increase even with
a small temperature increase and will become greater at higher temperatures
(high confidence). Extreme events include, for example, floods, soil moisture
deficits, tropical cyclones, storms, high temperatures, and fires. The impacts
of extreme events often are large locally and could strongly affect specific
sectors and regions. Increases in extreme events can cause critical design or
natural thresholds to be exceeded, beyond which the magnitude of impacts increases
rapidly (high confidence). Multiple nonextreme consecutive events also can be
problematic because they can lessen adaptive capacity by depleting reserves
of insurance and reinsurance companies. [8, 19.6.3.1]

An increase in the frequency and magnitude of extreme events would have adverse
effects throughout sectors and regions. Agriculture and water resources may
be particularly vulnerable to changes in hydrological and temperature extremes.
Coastal infrastructure and ecosystems may be adversely affected by changes in
the occurrence of tropical cyclones and storm surges. Heat-related mortality
is likely to increase with higher temperatures; cold-related mortality is likely
to decrease. Floods may lead to the spread of water-related and vector-borne
diseases, particularly in developing countries. Many of the monetary damages
from extreme events will have repercussions on a broad scale of financial institutions,
from insurers and reinsurers to investors, banks, and disaster relief funds.
Changes in the statistics of extreme events have implications for the design
criteria of engineering applications (e.g., levee banks, bridges, building design,
and zoning), which are based on estimates of return periods, and for assessment
of the economic performance and viability of particular enterprises that are
affected by weather. [19.6.3.1]

7.2.5. Large-Scale Singular Events

Human-induced climate change has the potential to trigger large-scale changes
in Earth systems that could have severe consequences at regional or global scales.
The probabilities of triggering such events are poorly understood but should
not be ignored, given the severity of their consequences. Events of this type
that might be triggered include complete or partial shutdown of the North Atlantic
and Antarctic Deep Water formation, disintegration of the West Antarctic and
Greenland Ice Sheets, and major perturbations of biosphere-regulated carbon
dynamics. Determining the timing and probability of occurrence of large-scale
discontinuities is difficult because these events are triggered by complex interactions
between components of the climate system. The actual discontinuous impact could
lag the trigger by decades to centuries. These triggers are sensitive to the
magnitude and rate of climate change. Large temperature increases have the potential
to lead to large-scale discontinuities in the climate system (medium confidence).

These discontinuities could cause severe impacts on the regional and even global
scale, but indepth impact analyses are still lacking. Several climate model
simulations show complete shutdown of the North Atlantic thermohaline circulation
with high warming. Although complete shutdown may take several centuries to
occur, regional shutdown of convection and significant weakening of the thermohaline
circulation may take place within the next century. If this were to occur, it
could lead to a rapid regional climate change in the North Atlantic region,
with major societal and ecosystem impacts. Collapse of the West Antarctic Ice
Sheet would lead to a global sea-level rise of several meters, which may be
very difficult to adapt to. Although the disintegration might take many hundreds
of years, this process could be triggered irreversibly in the next century.
The relative magnitude of feedback processes involved in cycling of carbon through
the oceans and the terrestrial biosphere is shown to be distorted by increasing
temperatures. Saturation and decline of the net sink effect of the terrestrial
biospherewhich is projected to occur over the next centuryin step
with similar processes, could lead to dominance of positive feedbacks over negative
ones and strong amplification of the warming trend. [19.6.3.2]